1. Field of the Invention
This invention relates to power supply switching circuits and particularly, although not exclusively, to such circuits for use with nuclear event detectors.
2. Discussion of Prior Art
It is known that exploding nuclear weapons produce pulses of ionising radiation which can damage electronic components. To minimise the damage, it is necessary to shut down the power supply to the components as rapidly as possible. Known ways of achieving this involve the use of a nuclear event detector coupled to a protection circuit. For example, in response to an ionising pulse of radiation, the nuclear event detector generates a pulse which activates a power supply crowbar circuit. After a short period, when the components have recovered, the power is reapplied.
If the nuclear event detector (NED) shares the same power supply as the electronic components then a means for powering the NED during the time-out period is required. This is conventionally done using local storage capacitors. In addition there is a need for a switching circuit for isolating the storage capacitors from the power supply during the time-out period so that no leakage of current can occur. In addition the switching circuit must provide an adequate power supply to the NED during standby mode and maintain the NED supply at a voltage of at least 4.5V (assuming the NED component requirements are a nominal 5 Volts) during its operation.
The voltage drop of any circuit associated with supply isolation is a critical issue with conflicting requirements. For example, the higher the supply voltage, the higher is the power dissipation during standby. However, the lower the supply voltage, the less is the voltage drop that can be tolerated by the isolation circuit during standby. Another consideration is that the lower the NED supply (ie the supply voltage minus the voltage drop across the isolation circuit), the larger the storage capacitors need to be.
Maxwell Laboratories Inc. Application Note of 1986 describes two alternative power switching circuits for a nuclear event detector.
A first circuit relies on the use of a diode for isolating the local storage capacitance from a nominal 5 Volt power supply during the time-out period. However, because of the high forward voltage drop of the diode, this circuit must be operated from a power supply voltage approximately one diode drop (typically 0.5v to 1v) higher than the usual 5 volts required; which can be a disadvantage. Furthermore, as the voltage drop across the diode is comparatively high, the storage capacitors need to be quite large in order to compensate.
The second circuit proposed in the aforementioned Application Note employs a series pass transistor for isolation, which provides a low voltage drop which is largely independent of temperature. However, this configuration is believed to be flawed because when the power supply voltage is removed, the transistor becomes biased in the reverse sense (ie the collector behaves as the emitter and vice versa) and consequently bleeds current from the storage capacitors.